Monolog FingersCrossedHandler

This was changed in version 1.11.0 of Monolog. There's now an optional 6th parameter, $passThruLevel. This parameter is the minimum level log that should always be flushed. In your case, you should set up FingersCrossed in the following manner:

use Monolog\Handler\FingersCrossedHandler;

$fingersCrossedHandler = new FingersCrossedHandler(
    $streamHandler,
    Monolog\Logger::ERROR,
    0,
    true,
    true,
    Monolog\Logger::INFO
);

This will result in INFO or higher messages always being logged, but DEBUG messages will only show up if an ERROR level or higher message has been logged.

This is not possible with the FingersCrossedHandler as such. You could easily extend it though and override handle() so that it buffers only debug messages and lets the rest through always. Problem is if you do this you'll have messages out of order when an error occurs, unless you buffer everything, and at the end flush it all except debug if no error occurred.